Because of their outstanding thermal conductivity, precipitation hardened and dispersion strengthened copper-based alloys are currently of interest for use in gas and water-cooled composite-metal parts for high temperature gas turbines. These composite-metal parts, including water-cooled buckets and nozzles for example, preferably comprise a copper alloy core having a second, more corrosion resistant metal cladding metallurgically bonded to the exposed, outer surface. The cladding permits the use of copper alloys at metal temperatures previously too high for copper-based materials, i.e., 600.degree. F. and above.
In this temperature range, however, existing high strength wrought copper alloys suffer a drastic reduction in strength due to recovery, recrystallization, and grain growth and they have not, therefore, provided the long term reliability necessary for gas turbine service. Although other, more stable copper alloys, such as the oxide dispersion strengthened systems (strengthened for example by Al.sub.2 O.sub.3,MgO,Zr.sub.2 O.sub.3) have good elevated temperature strength and stability they are difficult to produce and they exhibit marginally low stress-rupture ductility. In addition, other solid solution strengthened copper alloys such as Cu-Zn, Cu-Ni and Cu-Sn do not exhibit the high level of thermal conductivity necessary for effective water-cooling.
It is known in the art to strengthen nickel-based alloys for gas turbine service by a process of directional solidification. For example U.S. Pat. No. 3,260,505 discloses a turbine blade of nickel alloy which is strengthened in this manner. U.S. Pat. No. 3,494,709 further discloses alloy metal parts strengthened by this process for gas turbine service. However, it is notable that nickel and alloys thereof have relatively poor thermal conductivity, i.e., on the order of one-tenth or less the value for pure copper and precipitation hardened copper-based alloys. For this reason, alloys of nickel are not effective for gas or water-cooled turbine parts wherein the heat transferred from the high temperature turbine gas must be quickly and efficiently transferred to the cooling gas or water. Copper alloys are recognized as being the most effective and economical metals from a heat transfer standpoint.
It is, therefore, one object of the present invention to provide copper-based alloy articles having improved stress-rupture properties at elevated temperatures which can be manufactured without substantial sacrifice in alloy thermal conductivity.
Furthermore, recognizing that component parts of gas turbines are subject to operating stresses whose magnitudes are greater in some directions than in others, it is a principal object of the invention to provide copper-based alloy components for gas turbines which have enhanced high temperature fatigue resistance and strength and which are directionally strengthened parallel to the direction of primary stress imposed during turbine operation.